I did some testing using an old EICO modulation meter and an oscilloscope and went backand forth between the original LED and the 1n4148. For some reason I couldn't get the scope to behave enough to give accurate modulation numbers but I was able to easily bury the needle on the modulation meter while the o-scope displayed minimal apparent distortion. Using a 1khz test tone the volume was turned way up with the modulation meter completely buried (reads up to 100%) above the 100% mark. With the meter buried I could turn up the volume even higher before I noticed overmodulation on the o-scope. In tests with music there is subjectively more headroom and the sound is cleaner when playing music at very high levels which averaged about 80% modulation on the meter. The overall impression is that there is more headroom with the 1n4148.

I was able to modulate to at least 100% with either diode. The real key for maximum modulation seems to be correctly setting the carrier level at about 50% of the power supply voltage, then tweaking it a bit more by using an o-scope or listening over the air, and setting the compressor as a limiter. I found settings that gave the kick drums some extra punch and an overall higher volume but with some pumping of the sound. I ended up using compressor 8:1, threshold -12, attack 1.7ms, decay 3.5ms, and integration 106ms. I found that using a threshold below -12 limited the maximum modulation possible. Below about -16.5 I wasn't able to get to 100% modulation with a 1 khz tone.

Subjective tests with music seemed to prove that the sound was generally cleaner at very high modulation levels with the new diode giving the impression of a bit more headroom.

Sorry, i don't have any exact numbers...

Thu Oct 20, 2005 5:26 am

Music RadioNew registered user

Joined: 16 Oct 2005Posts: 2

Well, your on the right track using an O-scope.

Maybe Mark can do some testing also. It is quite as easy thing to check.

First you need to use a AM broadcast type processor--similar to the Inovonics 235 that has adjustable negative and positive modulation controls. The negative is with a 1k tone and adjust the level so that the waveform just about kisses for 100%. For the positive you have to use low compressed music or talking, a single tone will not show you the correct waveform. You will very easily see the positve peaks on the scope when you advance the positive peak control. The cool thing is this has no effect on the negative peak. I don't have the AM-MAX transmitter yet----however my xmtr is only capable of about 110% positive. So if Mark does a bit of checks with his scope. Tell us what this modification will do for the positive modulation at various carrier levels.

Thu Oct 20, 2005 6:02 am

pcsSite master

Joined: 18 Jan 2002Posts: 3086Location: Radio Land

I was seeing about 120% on my test unit, but I don't see why
it couldn't be more.

What this diode does is limit negative mod peaks.
You than move the carrier so that this limited value just
about "touches" the zero. Positive peaks are not limited.

Experimentation is required. Setting compression ratio
to less than 8:1 is probably beneficial to allow for positive
peaks. Maybe 2:1 or so.

Another mod for entusiasts, replace DU1 (100uH in PWM filter)
with a standard torroid-style 100uH/4A type. They are used in
things such as household light regulators or PWM power supplies.

Also remove any additional 220nF or 470nF caps in the PWM section
on the underside of the PCB.

Of course, if you buy AM MAX today we do this for you.
This mod improves audio quality.

I replaced the diode, put in a nice big 100uh 4.4A toroid, and took out the extra caps in my AM MAX II for 530-1710 and fired it up. For some reason the power level seems to ramp up for at least a minute or two before slowly stabilizing. Initial sound impressions are pretty good. It seems to have a beefier sound but I haven't done any A/B comparisons with my other AM MAX II yet. More testing to be done.

Any ideas on the power ramp up time?

Tue Nov 01, 2005 3:59 pm

AM-EnthusiastNew registered user

Joined: 26 Aug 2004Posts: 12

Just performed the mods on dual band model

I just replaced the diode and added the Toroid on the dual band AM Max II and it's sounding great. It's the best sound and modulation I've gotten out of one of these yet...